<template>
    <ul class="list">
        <li 
            class="item" 
            v-for="item of letters" 
            :key="item"
            :ref="item"
            @touchstart="handleTouchStart"
            @touchmove="handleTouchMove"
            @touchend="handleTouchEnd"
            @click="handleLetterClick"
        >
            {{item}}
        </li>
    </ul>
</template>

<script>
export default {
    name:"CityAlphabet",
    props:{
        cities:Object
    },
    computed:{
        letters() {
            const letters=[]
            for(let i in this.cities) {
                letters.push(i)
            }
            return letters  // ['A','B','C']
        }
    },
    data () {
        return {
            touchStatus:false,
            startY:0,
            timer:null
        }
    },
    updated () {
        this.startY = this.$refs['A'][0].offsetTop
    },
    methods:{
        handleLetterClick (e) {
            this.$emit('change',e.target.innerText)
        },
        handleTouchStart () {
            this.touchStatus=true
        },
        handleTouchMove (e) {
            if(this.touchStatus) {
                if(this.timer) {
                    clearTimeout(this.timer)
                }
                this.timer=setTimeout(() => { // 函数截流（通过延时器来减少函数连续的执行）
                    const touchY = e.touches[0].clientY - 79
                    const index = Math.floor((touchY-this.startY)/20)
                    if(index>=0 && index<this.letters.length) {
                        this.$emit('change',this.letters[index]) 
                    }
                },16)  
            }
        },
        handleTouchEnd () {
            this.touchStatus=false
        }
    }
}
</script>

<style lang="stylus" scoped>
    @import '~style/varibles.styl'

    .list
        display flex
        flex-direction column
        justify-content center
        position absolute
        top 1.58rem
        right 0
        bottom 0
        width .4rem
        .item
            line-height .4rem
            text-align center
            color $bgColor
</style>
